Process for removal of solid nonifibrous material from pulp

ABSTRACT

The present invention relates to a process for removal of solid non-fibrous material from an aqueous pulp suspension comprising providing an aqueous pulp suspension comprising solid non-fibrous material, removing at least part of the solid non-fibrous material from the aqueous pulp suspension thereby forming an aqueous pulp suspension depleted in solid non-fibrous material, dewatering the aqueous pulp suspension depleted in solid non-fibrous material thereby forming a web comprising cellulose-containing fibres and a filtrate comprising residual solid non-fibrous material removing at least part of the residual solid non-fibrous material from the filtrate thereby forming a filtrate depleted in residual solid non-fibrous material and an aqueous stream comprising at least part of the removed residual solid non-fibrous material and diluting the aqueous pulp suspension and/or the aqueous pulp suspension depleted in solid non-fibrous material with at least part of the filtrate depleted in residual solid non-fibrous material, whereby the aqueous stream comprising at least part of the removed residual solid non-fibrous material is purged from the system.

The present invention relates to a process for removal of solid non-fibrous material from an aqueous pulp suspension and to pulp comprising less than about 50 ppm of silica-based material obtained by the process.

Cellulosic pulp can be obtained by various processes such as processes where the cellulosic fibres of wood are separated by a chemical process or processes including mechanical means for separating the cellulosic fibres. The former processes are often referred to as chemical pulping processes and the latter as mechanical pulping processes. As to mechanical pulping processes such processes may also comprise steps comprising partly separating the cellulosic fibres by chemical treatment, alternatively, subjecting the wood, such as wood chips, to treatment stages facilitating the separation of cellulosic fibres in a subsequent mechanical treatment stage. Mechanical pulping processes comprising additional non-mechanical process steps may be referred to as thermomechanical pulping processes, chemimechanical pulping processes and chemithermomechanical pulping processes. The main constituents of wood and some non-wood materials are cellulose, hemicelluloses and lignin. In a chemical pulping process, wood, typically wood chips, is subjected to a chemical process stage where wood chips are mixed with various chemicals and subjected to elevated temperatures and super atmospheric pressure. During this process stage, often referred to as the cooking stage, and operated in large pressure resistant vessels (digesters) the lignin is chemically removed from the wood material without significantly degrading the cellulose (such as cellulose and hemicelluloses). Accordingly, during the cooking process the content of lignin of the wood material is gradually decreased down to a region depending on the usage of the pulp and various other factors including type of cooking process and optional bleaching sequence. In many applications the pulp must have a high brightness. Brightness of pulp is highly dependent on the amount of lignin in the pulp. A pulp with a high brightness calls for a low amount of lignin. As already indicated the content of lignin is gradually reduced during the cooking process, however, also the cellulose is degraded during said process and becomes more pronounced as the content of lignin diminishes. Thus, the operation and termination of the cooking process is significantly based on the amount of residual lignin present in the wood material/cellulosic fibre material and the residual lignin contain is further reduced in the bleaching process provided pulp with high brightness is required. After bleaching, the pulp may be used in a paper mill which can be integrated with the pulping (pulp) mill. Alternatively, the bleached pulp suspension may be dewatered and subsequently dried to a format facilitating transportation to the location of further processing, such as a paper/paper board mill. In a measure of making the pulping process and paper making process more economically and environmentally viable, the processes are more and more closed such that spent chemicals and water are recovered and re-circulated. Water from e.g. the wire is to an extent re-circulated. The increasing closure of the pulping process (pulp mill) and the paper manufacturing process (paper mill) leads to the accumulation of solid material in the system such as solid silica-based material.

Thus, it is an object of the present invention to provide a process for the removal of solid non-fibrous material such as solid silica-based material from aqueous suspensions comprising cellulosic fibres.

While a share of bleached pulp is used in the manufacture of paper and tissue bleached pulp may also be used for the production of textiles such as rayon and cellophane and as a raw material for the production of cellulose derivative including carboxyalkyl substituted cellulose. Typically, pulps having a high content of cellulose (such as above about 92% based on total pulp) and, optionally also high brightness, are used for the above mentioned non-paper/non-tissue materials. In order to obtain a pulp with a high content of cellulose other non-cellulose constituents of the wood are usually removed, specifically the hemicellulose. The hemicellulose of wood is usually removed prior to chemical digestion by acid hydrolysis. Pulp with a high content of cellulose based on total pulp is often referred to as dissolving pulp. Many processes using pulps having a high content of cellulose are improved if the amount of solid non-fibre materials, specifically silica-containing materials, is kept low. It is beneficial to have low contents of solid silica-based material in the pulp while manufacturing textile fibres such as rayon.

Thus, it is an objective with the present invention to provide a process for the removal of solid non-fibrous material, such as solid silica-based material, from an aqueous pulp suspension comprising cellulosic fibres, e.g. a pulp having a high content of cellulose, notably a dissolving pulp.

A further object of the invention is to provide a process which is capable of removing solid non-fibrous material, such as silica-based material from an aqueous pulp suspension comprising cellulosic fibres in an energy efficient manner.

A still further object of the invention is to efficiently remove solid non-fibrous material having a very small particle size, such as an average particle size of less than about 100 um.

Still further objects are apparent from the application below.

U.S. Pat. No. 7,364,640B2 discloses a process for removing silica from nonwood plant materials involving mechanical and chemical action.

U.S. Pat. No. 6,190,504 relates to a method in a paper machine for arrangement of water circulation and arrangement of a water circulation system in a paper machine.

WO 2008067916 A1 relates to a method of treating filtrate produced in thickening of paper fibre suspension. One objective with the method is to reduce the loss of fine particles which is achieved by recycle back fine particles (13) separated from the filtrate (F6) downstream.

The objects of the invention can be achieved by a process as defined by the claims.

The present invention relates to a process for removal of solid non-fibrous material from an aqueous pulp suspension comprising providing an aqueous pulp suspension comprising solid non-fibrous material, removing at least part of the solid non-fibrous material from the aqueous pulp suspension thereby forming an aqueous pulp suspension depleted in solid non-fibrous material (yet further comprising residual solid non-fibrous material) and also typically an aqueous stream comprising at least part of the solid non-fibrous material (may be referred to as first stream, H1A), dewatering the aqueous pulp suspension depleted in solid non-fibrous material thereby forming a web comprising cellulosic fibres and a filtrate comprising residual solid non-fibrous material, the filtrate typically also comprising cellulose-containing fibres, removing residual solid non-fibrous material from the filtrate thereby forming a filtrate depleted in residual solid non-fibrous material and an (further) aqueous stream comprising at least part of the removed residual solid non-fibrous material (may be referred to as second stream, H2A); and diluting the aqueous pulp suspension and/or the aqueous pulp suspension depleted in solid non-fibrous material with at least part of the filtrate depleted in residual solid non-fibrous material, whereby the aqueous stream comprising at least part of the removed residual solid non-fibrous material is purged from the system. The present invention also encompasses pulp comprising less than about 50 ppm of silica-based material (calculated as SiO₂) by weight based on total oven dry pulp obtainable by the present process.

An embodiment of the present invention relates to a process for removal of solid non-fibrous material from an aqueous pulp suspension comprising providing an aqueous pulp suspension comprising solid non-fibrous material, removing at least part of the solid non-fibrous material from the aqueous pulp suspension thereby forming an aqueous pulp suspension depleted in solid non-fibrous material and an aqueous stream comprising at least part of the solid non-fibrous material (H1A), dewatering the aqueous pulp suspension depleted in solid non-fibrous material thereby forming a web comprising cellulose-containing fibres and a filtrate comprising residual solid non-fibrous material removing at least part of the residual solid non-fibrous material from the filtrate thereby forming a filtrate depleted in residual solid non-fibrous material and an aqueous stream comprising at least part of the removed residual solid non-fibrous material (H2A), and diluting the aqueous pulp suspension and/or the aqueous pulp suspension depleted in solid non-fibrous material with at least part of the filtrate depleted in residual solid non-fibrous material, whereby the aqueous stream comprising at least part of the solid non-fibrous material (H1A) and the aqueous stream comprising at least part of the removed residual solid non-fibrous material are purged from the system.

According to a preferred embodiment, the filtrate depleted in residual solid non-fibrous material is used to dilute the aqueous pulp suspension prior to the removal of solid non-fibrous material from the aqueous pulp suspension.

According to an embodiment the invention relates to a process for removal of solid non-fibrous material from an aqueous pulp suspension comprising cellulose-containing fibres, comprising providing an aqueous pulp suspension comprising solid non-fibrous material and cellulose-containing fibres, a first means for removing solid material from an aqueous suspension (first removal means), a second means for removing solid material from an aqueous suspension (second removal means), means for dewatering the aqueous pulp suspension comprising cellulose-containing fibres, the process comprising removing at least part of the solid non-fibrous material from the aqueous pulp suspension by the first removal means thereby forming an aqueous pulp suspension depleted in solid non-fibrous material (but comprising residual solid non-fibrous material), dewatering the aqueous pulp suspension depleted in solid non-fibrous material by the dewatering means thereby forming a web of cellulose-containing fibres and a filtrate comprising residual solid non-fibrous material (and typically cellulose-containing fibres), removing at least part of the residual solid non-fibrous material from the filtrate by the second removal means thereby forming a filtrate depleted in residual solid non-fibrous material and an aqueous stream comprising at least part of the removed residual solid non-fibrous material; and diluting the aqueous pulp suspension and/or the aqueous pulp suspension depleted in solid non-fibrous material with at least part of the filtrate depleted in residual solid non-fibrous material, whereby the aqueous stream comprising at least part of the removed residual solid non-fibrous material is purged from the system.

FIG. 1 is a schematic view of an embodiment of the process according to the invention.

FIG. 2 is a schematic view of a process (not according to the present invention) where the residual solid non-fibrous material is not removed from the filtrate.

The term aqueous pulp suspension is used to denote an aqueous cellulose-containing fibre suspension further comprising solid non-fibrous material. Cellulose-containing fibres may also be referred to as lignocelluloses-containing fibres as residual lignin usually is present. In the present process, and also when the process is integrated within another fibre process including a pulping process or process for recycling used cellulose-containing fibres, the definition aqueous pulp suspension relates to the main cellulose-containing fibre flow. The term pulp is used to denote a cellulose-containing fibrous material obtained from wood, non-wood sources, recycled fibres, and mixtures thereof. According to an embodiment of the invention pulp is obtained from wood, or recycled fibres, or mixtures thereof. In addition to cellulose pulp usually also contains hemicellulose, and residual lignin, as well as other fibrous and non-fibrous constituents albeit in a minor amount, such as below about 20% w/w based on total solids (fibre and other solid material). According to established practice the amount of lignin on the pulp may be expressed by the Kappa number. Within the context of this application the kappa number is established according to the Standard Method SCAN-C 1:77. Preferably, the pulp is obtained from wood such as hardwood and softwood. As already mentioned above, the pulp may be obtained by a chemical or mechanical pulping process. According to an embodiment, the pulp is obtained by subjecting cellulose-containing material to a chemical cooking process including the sulphate, sulphite, soda and organosolv process, i.e. chemically digested pulps. Chemically digested pulp can also be referred to as a chemical pulp. The chemical cooking process is preferably chosen among the sulphate process and sulphite process. Thus, the pulp may be chosen among sulphate pulp, sulphite pulp, soda pulp, organosolv pulp, and mixtures thereof, more preferably the pulp may be chose among sulphate pulp and sulphite pulp, or, the pulp is a sulphate pulp. Cooking of softwood using the sulphate process is preferable, i.e. sulphate pulp obtained from softwood. According to yet another embodiment the pulp is obtained by the sulphate or sulphite process, the processes also comprising subjecting wood, suitably wood chosen among softwood and hardwood, to a chemical treatment stage comprising the removal of hemicellulose prior to the chemical digestion rendering pulp with a content of cellulose above about 85% by weight based on total oven dry pulp, suitably above about 90%, above about 92%. Thus, according to an embodiment the pulp has a content of cellulose of above about 85% by weight, preferably above about 90%, above about 92%, such as above about 95% by weight based on total oven dry pulp. Pulps obtained by a pulping process comprising chemical digestion and a stage where hemicelluloses is removed, are usually referred to as dissolving pulps. Thus, according to an embodiment the pulp is a dissolving pulp.

In order to further reduce the content of lignin and increase the brightness of the pulp after chemical cooking process in the digester, the pulp is subjected to a bleaching process. Usually, the bleaching process comprises a multistage process comprising several bleaching stages which usually are separated by washing stages. The content of lignin in the pulp gradually decreases through the bleaching process, whereas the brightness of the pulp increases. According to an embodiment of the invention the pulp, such as pulp having a cellulose content as disclosed above, such as a cellulose content above about 85%, above about 90% , above about 92%, has a brightness of above about 60% (ISO Brightness), above about 80% ISO, suitably above about 90% ISO. According to a further embodiment, the pulp is dissolving pulp, suitably with a brightness of above about 90% ISO, and optionally with a content of cellulose above about 80% by weight based on total oven dry pulp, such as above about 85%, or above about 90% w/w.

According to yet another embodiment the pulp has a kappa number of below about 35 such as below about 30, suitably below about 20.

According to yet another embodiment, at least part of the cellulose-containing fibres comprised in the pulp of the aqueous pulp suspension may originate from recycled paper or board or mixtures thereof, i.e. non-virgin cellulose-containing fibres. The pulp may also essentially consist of cellulose-containing fibres originating from recycled cellulose containing products (such as paper and board). For example, the pulp may originate from old corrugated containers (OCC). Pulp originating from recycled paper and board (such as OCC) contain an amount of solid non-fibrous material such as stickies, waxes, hot smelts. When recycling paper and board, i.e. re-using the fibre material (cellulose-containing fibres), it is beneficial if non-fibrous materials are removed prior to processing the obtained recycled fibre material on order to produce novel paper or board. The present method can preferably be applied for removing solid non-fibrous materials from pulp originating from paper and board, recycled pulp.

The consistency of the non-diluted aqueous pulp suspension comprising solid non-fibrous material and cellulosic fibres is typically at least about 2.0% by weight of oven dry matter to total weight of pulp suspension, typically from about 2.0% by up to about 5.0% by weight of oven dry matter to total weight of pulp suspension, suitably from about 2.5 up to about 4.0%, and more preferably from about 2.5 up to about 3.5%. Within the context of this application consistency is defined as the weight percentage of oven-dry matter accounting for the total weight of aqueous composition, e.g. aqueous pulp suspension or filtrate. The consistency in this application is calculated according to TAPPI T 240 om-07. According to an embodiment of the present invention the present process for removal of solid non-fibrous material from an aqueous pulp suspension comprising cellulosic fibres is integrated in a process selected from the group consisting of a pulping process or a process for recycling fibres. According to a further embodiment the present process is integrated in a pulping process. The aqueous pulp suspension, the aqueous pulp suspension depleted in solid non-fibrous material, and the web comprising cellulose-containing fibres represent the (main) fibre line. When the present process is integrated within a pulping process the present process is commonly situated after the bleaching process and typically after the final pulp storage tower. According to the invention at least part of the solid non-fibrous material comprised in the aqueous pulp suspension is removed by subjecting the aqueous pulp suspension to a first means for removal of solids from an aqueous suspension. The first means for removal of solids may be any means which is capable of effectively separating solids, specifically solid-non fibrous material, from an aqueous pulp suspension. Typically, the first means is capable of removing solid non-fibrous particles which would be retained in the web comprising cellulose-containing fibres after the dewatering operation. The first means for removal may be based on size exclusion e.g. filters, screens, or separation/removal means based on centripetal force and fluid resistance, such as a multistage hydrocyclone system, or any combination of means based on size exclusion and on centripetal force and fluid resistance. According to an embodiment the removal means is a process separating solid material from a liquid based on the ratio of centripetal force to fluid resistance, such as a multistage hydrocyclone system. Due to the consistency of the aqueous pulp suspension, i.e. the content of fibres in the aqueous pulp suspension, solid non-fibrous material having a small particles size, typically solids having an average particle size of less than 150 μm, is not effectively removed by the first means. Thus, the aqueous pulp suspension subjected to the first removal means and depleted in solid non-fibrous material still contains residual solid non-fibrous material. After dewatering the aqueous pulp suspension, suitably on a wire, a web comprising cellulose-containing fibres is obtained as well as a filtrate said filtrate comprising residual solid non-fibrous material. The residual solid non-fibrous material mainly follows the filtrate while the fibres are mostly retained in the web. The consistency of the filtrate is dependent on several factors such as the aqueous pulp suspension and type of dewatering process. Due to the dewatering of the aqueous pulp suspension the consistency of the filtrate is lower or significantly lower, than the consistency of the aqueous pulp suspension. Suitably, the consistency of the filtrate is preferably less than about 0.25% by weight of oven-dry matter accounting for the total weight of aqueous composition, such as less than about 0.1%, less than about 0.05%, and suitably from about 0.01% up to about 0.25% by weight, preferably from about 0.01 up to about 0.1% by weight. According to the invention at least part of the residual solid non-fibrous material is removed by subjecting at least part of the filtrate to a second means for removing solids from an aqueous suspension. According to an embodiment the second means for removing solids from an aqueous suspension is a process separating solid material from a liquid based on the ratio of centripetal force to fluid resistance, suitably a multistage hydrocyclone system. As the consistency of the filtrate is low and lower than the consistency of the aqueous pulp suspension, the aqueous pulp suspension optionally diluted with the filtrate depleted in residual solid non-fibrous material, the second means can be designed to more efficiently remove solid non-fibrous material not being removed by the first means, and referred to as residual solid non-fibrous material. Typically, the second removal means is capable of removing solid non-fibrous particles not removed by said first removal means. The average particle size of the residual solid non-fibrous material is typically smaller than the average particle size of the solid non-fibrous material removed by the first means. While the first means may effectively remove solid non-fibrous material having an average particle from about 500 μm, or suitably from about 200 μm, the second means effectively removes residual solid non-fibrous material having an average particle size of below about 500 μm, suitably below about 200 μm, below about 100 μm, and below about 50 μm.

After the removal of at least part of the residual solid non-fibrous material from the filtrate at least part of the filtrate depleted in residual solid non-fibrous material is used to dilute the aqueous pulp suspension (thereby forming a diluted aqueous pulp suspension) or the aqueous pulp suspension depleted in solid non-fibrous material. Furthermore, in accordance with the invention the aqueous stream comprising at least part of the removed residual solid non-fibrous material is purged from the system. The aqueous stream may also (H1A and H2A) may also be referred to as reject, or alternatively, aqueous reject. According to an embodiment the aqueous stream comprising at least part of the solid non-fibrous material (H1A) and the aqueous stream comprising at least part of the removed residual solid non-fibrous material are both purged from the system. By purged from the system is meant that the aqueous streams comprising at least part of the solid non-fibrous material and/or the removed residual solid non-fibrous material is not in any way re-cycled back to the system. According to a further embodiment, the aqueous stream comprising at least part of the removed residual solid non-fibrous material (H2A) is not fed to a location downstream the dewatering of the aqueous pulp suspension depleted in solid non-fibrous material. According to yet a further embodiment the aqueous stream comprising at least part of the solid non-fibrous material (H1A) and also the aqueous stream comprising at least part of the removed residual solid non-fibrous material (H2A) are not fed to a location downstream the dewatering of the aqueous pulp suspension depleted in solid non-fibrous material. According to a preferred embodiment at least part of the filtrate depleted in residual solid non-fibrous material is used to dilute the aqueous pulp suspension prior to the diluted aqueous pulp suspension is subjected to the first removing means. Optionally, the filtrate depleted in residual solid non-fibrous material may, in addition to being used as diluents for the aqueous pulp suspension, also be added to the aqueous pulp suspension depleted in solid non-fibrous material. The amount/volume of filtrate used for diluting the aqueous pulp suspension is dependent on several factors including but not limited to the consistency of the aqueous pulp suspension prior to dilution and the target consistency of the pulp suspension just prior to dewatering. The consistency of the aqueous pulp suspension when subjected to the first removal means is partly dependent on the type of fibres (e.g. fibre length) and type of first removal means. Typically, the consistency of the aqueous pulp suspension when subjected to the first removal means, typically after dilution with the filtrate depleted in residual non-fibrous material, is the range of from about 0.5 up to about 3.0%, suitably from about 1.0 up to about 2.0%.

According to an embodiment said first and second means for removing solid non-fibrous material relate to processes separating solid material from a liquid which is based on the ratio of centripetal force to fluid resistance. According to a further embodiment, the first and second means are multistage hydrocyclone systems. Thus, the first and second removal means may be defined as the first and second multistage hydrocyclone systems. The term multistage hydrocyclone systems encompass systems comprising several stages where each stage contains a multitude of hydrcyclones operated in parallel and in forward mode. Each stage may contain up to several hundred single hydrocyclones. Normally, a hydrocyclone has two exits arranged along an axis and a fluid inflow perpendicular to said axis. Typically, the hydrocyclone comprises a cylindrical section and a conical section (base) where said fluid inflow is fed tangentially into the cylindrical section. Within the fluid (such as an aqueous pulp suspension or filtrate comprising solids, e.g. non-fibrous material) centrifugal forces are countered by resistance within the fluid effecting that larger or denser particles are transported to the wall of the hydrocyclone which eventually exit at the rejection side (conical side), often referred to as reject or underflow, while the finer, or less dense particles, remain in the liquid and exit at the overflow side of the cylindrical section, referred to as overflow or accept. According to a preferred embodiment the first and second multistage hydrocyclone systems are operated in a forward mode. More specifically, the aqueous pulp suspension, and filtrate comprising residual solid non-fibrous material, respectively, are feed to the first stage of a multistage hydrocyclone plant (system) operating in a forward mode creating an overflow from the first stage hydrocyclones, i.e. an aqueous depleted in solid non-fibrous material, and a filtrate depleted in residual solid non-fibrous material, respectively, and an underflow from the final stage hydrocyclones enriched in solid non-fibrous material, i.e. aqueous streams (H1A, H2A) enriched in solid non-fibrous material. Forward mode operation of a hydrocyclone usually means that particles of higher density (such as solid non-fibrous material) are enriched in the underflow while particles of lower density (such as cellulosic fibres) are enriched in the overflow together with most of the water.

Solid non-fibrous material relates to various types of solid material not including cellulose-containing fibres. The solid non-fibrous material may have an average particle size below about 1 mm, below about 0.5 mm, below about 300 μm, suitable below 250 μm, below about 200 μm, below about 100 μm, preferably below about 50 μm. The process is capable of removing solid non-fibrous material in the colloidal range, i.e. solid non-fibrous material having an average particle size below about 50 μm, such as in the range from about 1 up to about 30 μm. The solid non-fibrous material can be any solid non-fibrous material including inorganic material such as silica-based material/particles and solid non-fibrous material originating from re-pulping recovered paper and paper board such as sticky material like glue residues, adhesives and printing ink. According to an embodiment the non-fibrous material is solid silica-based material. Solid silica-based material may be defined as silica-based material. The silica-based material may originate from the wood per se or stem form added chemicals, such as cooking chemicals, bleaching agents, and chemicals used in conjunction with the paper making process such as various silica-based compounds, e.g. silicic acid, and silica-based polymers. Silica-based material includes inorganic compounds comprising silica such as silica (silicon dioxide) in crystal or amorphous form. The solid non-fibrous material may also be characterised as a solid material having a specific (or relative) density which is higher than 1, wherein the specific density (SG) is defined as

${DS} = {\frac{{\partial{solid}}\mspace{14mu} {material}}{{\partial{fibrous}}\mspace{14mu} {material}\mspace{14mu} \left( {{cellulose}\mspace{14mu} {fibre}} \right)}.}$

As indicated above the process can be applied to the removal of solid non-fibrous material form a variety of different pulps. A pulping process may be designed so that different types of pulp can be produced as a response to changes in the market. For example, a pulping process may be capable of producing a specific quality of pulp during a given period of time, such as market pulp for paper or paper board, and, as a response to marker need, (partly) switch to the production of another quality of pulp, e.g. dissolving pulp. Many of the processes using dissolving pulp as raw material (e.g. production of textiles such as rayon) are negatively influenced by the presence of solid non-fibrous material, specifically silica-based material. The second removal means, typically a multistage hydrocyclone system, of the present process removes solid non-fibrous material from the filtrate, typically having a low consistency. Running the second multistage hydrocyclone system at a low consistency, signifying that the consistency of the inflow (i.e. the filtrate) is low, allows the multistage hydrocyclone system to be designed so as to be able to efficiently remove fine solid particles, such as solid non-fibrous material and typically solid silica-based material, typically having an average particle size of less than 50 μm. The ability of removing fine solid non-fibrous material may not be needed, such as during the production of market pulp, and for such pulp qualities the second means for removal of solid material can be bypassed. In this mode of operation solid non-fibrous material is removed in the first removal means. Accordingly, a further object of the present invention is the adaptability of the process to a variety of pulp qualities.

According to the present invention the consistency of the diluted aqueous pulp suspension, i.e. main fibre line, subjected to the first means of removal of solids is suitably from about 0.5 up to about 3.0%, and preferably from about 1 up to about 2%. In conventional processes the consistency of the main fibre line is lower, or significantly lower, signifying that the volume based on fibre is significantly higher for a conventional process that the volume of the main fibre line (aqueous pulp suspension) of the present process. The reduction in consistency of the main fibre line is needed for more efficiently removing aggregated solids in a conventional process, but, significantly increases the amount volume per weight of fibre translating in a significant increase of energy required for processing the same amount of fibre per time compared to the present invention. Thus, a further advantage of the invention is that energy consumption is reduced while at the same time fine solid non-fibrous material is more efficiently removed from the aqueous pulp suspension, and hence, from the pulping process. As a result a dewatered web comprising cellulose-containing fibres is obtained having a low amount of solid non-fibrous material, such as silica-based material, comprising less than about 50 ppm, suitably less than about 35 ppm, preferably less than 20 ppm of solid non-fibrous material, and even as low as less than about 10 ppm. If the solid non-fibrous material is silica-based material the the amount of silica-based material is calculated as SiO₂.

FIG. 1 shows an overview of a non-limiting embodiment of the process according to the invention. The main fibre line of the process is indicated by the bold line and may be integrated in a pulping process. Alternatively, the pulp of the aqueous pulp suspension (APS) may originate from recycled paper and board such as old corrugated containers (OCC). An aqueous pulp suspension (APS) having a consistency of around 3.0% is diluted to a consistency in the range of from about 1.5 to about 2.5% with the filtrate depleted in residual silica-based material (H2B) having a consistency of from about 0.01% up to about 0.25%. The diluted aqueous pulp suspension (DAPS) is fed to the first stage of a multiple stage hydrocyclone plant (H1) operating in a forward mode creating an overflow (H1B) from the first stage of hydrocyclones, i.e. the aqueous suspension depleted in silica based material (H1B) and an underflow (H1A) from the final stage hydrocyclones enriched in silica-based material. Some water is purged by the underflow, thus, the aqueous pulp suspension depleted in silica-based material (H1B) has a slightly increased consistency as compared to the diluted aqueous pulp suspension. The multiple stage hydrocyclone plant (H1) removes part of the silica-based material having an average particle size of suitably down to about 500 μm, however, fine silica-based material such as silica having an average particle size of less than 500 μm, less than 200 μm, or less than 100 μm, herein referred to as residual silica-based material, is still comprised in the aqueous pulp suspension depleted in silica-based material (H1B). The overflow (H1B), i.e. the aqueous pulp suspension depleted in silica-based material but still comprising residual silica-based material, is fed to a dewatering process which may comprise a headbox (HB) a wire (W). In the dewatering process a web of cellulose-containing fibres is formed together with a filtrate (F) (also referred to as white water) which filtrate is fed to a storage tank (ST). Most of the fine silica-based material (residual silica-based material) follows the filtrate (F). At least part of the filtrate (F) which has a consistency of from about 0.01% up to about 0.25% is fed to the first stage of a second multiple stage hydrocyclone plant (H2) operating in a forward mode creating an overflow (H2B) from the first stage hydrocyclones, i.e. a filtrate depleted in residual silica-based material, and an underflow (H2A) from the final stage hydrocyclones enriched in silica-based material. The second multiple stage hydrocyclone plant (H2) is designed so as to effectively removing/separating the residual fine silica-based material from the filtrate typically having an average particle size of less than 50 μm. The overflow (H2B) from the second multiple stage hydrocyclone plant (H2), is used to dilute the aqueous pulp suspension (APS). The web of cellulose-containing fibres is optionally dried. Furthermore, the aqueous pulp suspension (APS) is preferably obtained by diluting a pulp suspension having a consistency in the range of from 10 to 12% which comes from the last pulp storage tank of a pulping process to a consistency of 2.5 to 3.5%.

FIG. (2) shows a process where the filtrate flow over the second multiple stage hydrocyclone plant is by-passed. This mode of operation, which is not according to the invention, is preferably conducted when market pulp is produced. Hence, the process according to the invention is very flexible and can easily be adapted to a variety of pulp qualities. 

1. A process for removal of solid non-fibrous material from an aqueous pulp suspension comprising the steps of: providing an aqueous pulp suspension comprising solid non-fibrous material, removing at least part of the solid non-fibrous material from the aqueous pulp suspension thereby forming an aqueous pulp suspension depleted in solid non-fibrous material, dewatering the aqueous pulp suspension depleted in solid non-fibrous material thereby forming a web comprising cellulose-containing fibers and a filtrate comprising residual solid non-fibrous material, removing at least part of the residual solid non-fibrous material from the filtrate thereby forming a filtrate depleted in residual solid non-fibrous material and an aqueous stream comprising at least part of the removed residual solid non-fibrous material, and diluting at least one of the aqueous pulp suspension and the aqueous pulp suspension depleted in solid non-fibrous material with at least part of the filtrate depleted in residual solid non-fibrous material, whereby the aqueous stream comprising at least part of the removed residual solid non-fibrous material is purged from the system.
 2. The process according to claim 1, wherein the process is integrated in a pulping process.
 3. The process according to claim 1, wherein the pulp of the aqueous pulp suspension has a content of cellulose above about 85% by weight based on total oven dry pulp.
 4. The process according to claim 1, wherein the pulp has a kappa number of below about
 35. 5. The process according to claim 1, wherein the consistency of the aqueous pulp suspension is at least about 2.0% by weight based oven dry matter to total weight of pulp suspension.
 6. The process according to claim 1, wherein the concentration of solid non-fibrous material in the web of cellulose-containing fibers is below about 50 ppm by weight based on total weight oven dry web of cellulosic web.
 7. The process according to claim 1, wherein the consistency of the filtrate is below about 0.25%.
 8. The process according to claim 1, wherein the removal of residual solid non-fibrous material from the filtrate is achieved by a process separating solid material from a liquid based on the ratio of centripetal force to fluid resistance.
 9. The process according to claim 1, wherein the removal of solid non-fibrous material from the aqueous pulp suspension is achieved by a process separating solid material from a liquid based on the ratio of centripetal force to fluid resistance.
 10. The process according to claim 1, wherein the removal of solid non-fibrous material and residual solid non-fibrous material is achieved by a multistage hydrocyclone system.
 11. The process according to claim 1, wherein the average particle size of the solid non-fibrous material is below about 1 mm.
 12. The process according to claim 1, wherein the solid non-fibrous material is silica-based material.
 13. The process according to claim 1, wherein the consistency of the aqueous pulp suspension after dilution with the filtrate depleted in residual non-fibrous material is from about 0.5 up to about 3.0%, by weight of oven dry matter to total weight of pulp suspension.
 14. The process according to claim 1, wherein at least part of the pulp of the aqueous pulp suspension originates from recycled paper or board or mixtures thereof.
 15. Pulp comprising less than about 50 ppm of silica-based material, calculated as Si0₂, by weight based on total oven dry pulp obtainable by a process for removal of solid non-fibrous material from an aqueous pulp suspension comprising the steps of: providing an aqueous pulp suspension comprising solid non-fibrous material, removing at least part of the solid non-fibrous material from the aqueous pulp suspension thereby forming an aqueous pulp suspension depleted in solid non-fibrous material, dewatering the aqueous pulp suspension depleted in solid non-fibrous material thereby forming a web comprising cellulose-containing fibers and a filtrate comprising residual solid non-fibrous material, removing at least part of the residual solid non-fibrous material from the filtrate thereby forming a filtrate depleted in residual solid non-fibrous material and an aqueous stream comprising at least part of the removed residual solid non-fibrous material, and diluting at least one of the aqueous pulp suspension and the aqueous pulp suspension depleted in solid non-fibrous material with at least part of the filtrate depleted in residual solid non-fibrous material, whereby the aqueous stream comprising at least part of the removed residual solid non-fibrous material is purged from the system. 